The present invention relates to an electronic system for photographic devices, and is particularly applicable to cameras. The system provides metering, calculation and control of shutter speed and/or diaphragm aperture in response to certain input parameters and brightness of received illumination.
While not intending to be limited thereby, the background of the present invention and the concepts of the present invention will be described in the environment of a single lens reflex (SLR) camera. Such cameras typically include an objective lens, or interchangeable objective lenses, and a camera body to which the lens is attached. The camera body houses the film, shutter, and associated viewing optics and electronic control circuits for controlling either the speed of the shutter or the opening or aperture of the diaphragm. The diaphragm (iris) usually is contained within the objective lens assembly.
Various types of cameras of this nature presently are available on the market and typically two types of metering systems are used. One system includes a light sensor mounted in any of various manners behind the objective lens, and an electrical circuit which allows the user to select the appropriate shutter speed and aperture by changing either one or both while watching a meter pointer or other form of indicator in the viewfinder. These types of cameras frequently are referred to as having a "match-needle" exposure system.
Another form of camera of this nature has an "automatic" exposure system and is somewhat similar to the first but is more automatic in that the user selects either the aperture value or the shutter speed, and the other of these two variables is calculated by the electronic circuit and automatically controlled. Of these types of cameras, there is an "aperture-preferred" type wherein the user selects the f/stop, and the electronic system of the camera automatically computes and controls shutter speed, and a "shutter-preferred" type wherein the user selects shutter speed, and the electronic system automatically calculates and selects the appropriate diaphragm aperture value.
Early single lens reflex camera systems used stopdown metering wherein the light metering was performed with the diaphragm aperture set at the selected f/stop to ensure that the electronic circuit received the proper aperture value. This type of system had the disadvantage that the viewing brightness was diminished because the aperture was stopped down. Subsequently, SLR's were developed which allowed preview of the scene, which is going to be photographed, through an open or full aperture, and then metering through the selected stopped-down aperture.
Other types of automatic or semiautomatic cameras are available, many of which preceded the match-needle and automatic SLR's. In some of the former, generally a light sensor senses the incident light while the shutter is open and causes the shutter to close when sufficient light has been received for appropriate exposure of the film. In the typical SLR on the other hand, a hinged mirror is provided between the objective lens and the film plane, and serves to reflect an image of the subject to be photographed to the viewfinder. Then, when the shutter release button is depressed for taking the picture, the mirror films up thereby allowing light to pass through the objective lens and shutter to the film plane without any obstruction from the mirror. It was readily recognized by those in the photographic field that it was desirable to mount the light sensor in a manner which would not diminish the light reaching the film plane or otherwise obstruct the same. Accordingly, various systems were devised for either placing the light sensor in the viewing optical system following the mirror or providing a movable light sensor so that the same could be moved out of the path of light to the film when the film is being exposed. Examples of these arrangements are shown and described in U.S. Pat. No. 3,442,190 to Erickson.
Inasmuch as it has been desirable to provide metering of the incident light either during exposure or, at least, up to a time immediately preceding exposure, it also was readily apparent to those in the photographic field in designing single lens reflex cameras with either a light sensor in the viewing optics or a moving light sensor as noted above, that some means must be provided for storing an electrical signal proportional to the incident light so that proper exposure control could be accomplished even when the incident light no longer reached the light sensor at the time of exposure. This was readily accomplished by using the known storage or memory ability of a capacitor to receive the signal from the light sensor prior to exposure, to thereby memorize the light value, and then use the charge stored on the capacitor for shutter timing control. Numerous arrangements of this nature are disclosed in the literature, examples being shown in U.S. Pat. No. 3,324,779 and U.S. Pat. No. 3,695,157.
Further, in such systems of the latter type, it has been well established that the influence of the exposure determining factors of film speed (ASA), f/stop, and shutter speed on the exposure of the film varies in accordance with the variation of the exposure determining factors according to a geometrical progression having a common ratio of two. Because of this mathematical relationship, logarithmic circuits for logarithmically compressing and subsequently logarithmically expanding signals were used so that the necessary calculations could be performed with log signals in a relatively simple and straightforward manner. Examples of camera electronic system which describe such logarithmic circuits are found in U.S. Pat. No. 3,690,230 and No. 3,695,157. As explained in these latter two patents, the circuits using logarithmic compression and expansion require extremely complex circuits. In addition, while it is known that logarithmic conversion followed by calculation with the logarithmic signals involves simple addition and subtraction and thus is a desirable approach because the calculations are performed simply in a log scale and the data is compressed reducing the dynamic range requirement of the calculator, such logarithmic compression and expansion circuits can introduce significant errors because of drift, temperature variations, and the like. Furthermore, such prior systems have been limited in their calculation ability to calculate and display the shutter speed value or the diaphragm aperture value, and have been relatively complex because of mixing linear and log signals. Moreover, prior circuits freeze the controlled value (shutter speed or aperture) once the brightness value has been stored, thereby preventing a subsequent change in the control of the preferred value (the other one of shutter speed or aperture).